Get It Growing: Adding Soil? Make Sure You Get What Your Garden Needs

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Get It Growing News For 10/13/06

By Dan GillLSU AgCenter Horticulturist

I’m not a big fan of bringing in new soil and replacing the original soil in a garden bed. I think working to improve the existing soil and selecting plants that will grow well in the soil you have are the best things to do in most situations.

There are times, however, when it is necessary to purchase additional soil for the garden – especially when creating new raised beds or raising the grade of existing ones.

Usually, these purchased soils are sold as "topsoil" or "garden soil." You also may find soils available that are created especially for a particular type of plant, such as azalea soil or rose soil.

When you see the words topsoil or garden soil, though, you should be aware these materials are not literally what the names imply. The topsoil was not stripped off a field somewhere, and garden soil was not dug out of a garden. These materials actually are soil mixes that are created by blending together a variety of materials to produce a product plants will thrive in.

Selecting a topsoil or garden soil is very important. You need to be aware of what materials were blended together to create the mix you are purchasing.

For small jobs, it often is easiest to purchase bagged soil mix from a local nursery. Many nurseries carry soil mixes they have blended themselves or that they have obtained from a local company that makes soil mixes. You also will find bagged soils packaged by national companies, but these are not necessarily better, or even as good, as locally produced soil mixes.

For larger jobs, soil companies will sell you blended soil mixes by the cubic yard. Some will even deliver it if your order is large enough, or you can go directly to the company and pick it up yourself.

Soil companies generally use sand derived from various sources, such as local rivers and streams. The sand may be coarse or relatively fine, and it is blended with various organic materials in a variety of proportions to create the soil mixes. It is important to know what type of sand was used and its source. Some companies also may include silt (finer particles than sand) in their mixes.

Organic matter is a very important part of the mix and, in many cases, may provide the bulk of the mix. Organic matter comes in many forms such as manure, stable sweepings, bagasse (a byproduct of sugar milling), finely ground wood and rice hulls. All of these should be well composted before being used. Aged finely ground pine bark also is an excellent addition, especially for increased drainage. Fresh-ground pine bark, which has not been aged, and fly ash are less desirable.

I generally avoid soil mixes that contain fly ash. This material is essentially powdered charcoal and is added to soil mixes to make them look black – a color of soil gardeners generally associate with fertility. Fly ash, however, is strictly cosmetic and adds nothing to improve the soil. It is better to purchase soil mixes that get their dark brown color from the addition of quality composted organic matter.

You also should ask the company about the pH of their topsoil or garden soil, since this will have an important effect on plant growth. Ideally, the pH should be about 6 to 7.

Soil mixes often are used to fill new raised beds. Raised beds generally are built 8 inches to 12 inches high but may be higher. They are particularly useful if you are gardening in a situation where the drainage is poor.

When building a raised bed, construct the sides of the bed with your chosen materials, such as landscape timbers, bricks or pressure-treated boards. To decide how much soil you need to fill the bed, consult with the soil company or a nursery. Either can help you decide how much soil you need based on the dimensions of the bed (length times width time height).

Before adding the new soil, kill or remove any unwanted grass or weeds. Herbicides such as glyphosate are appropriate to use.

Once that’s done, till the soil in the bottom of the raised bed, add a few inches of the topsoil or garden soil and work it in. Then add enough new soil to reach about 2 inches to 3 inches below the edge of the sides. (This is important for proper watering and allows room to add mulch.)

If you add topsoil or garden soil to raise an existing bed, always try to blend it into the existing soil. Till the original soil. Add a few inches of the new soil and till it in. Then finish adding the new soil to the bed. This creates a transition between the new soil and the existing soil. Plants prefer this to a situation where there are distinct layers of different soils.

It usually is a good idea to dig in a fertilizer with about a 3:1:2 ratio (such as 15-5-10, 16-4-8 or anything similar) to the bed before planting. This will ensure there is sufficient nitrogen in the soil for the plants and will compensate for any nitrogen that may be tied up if the organic matter in the mix is still undergoing decomposition. (This is less of a problem if the organic matter in the mix was well composted before being added.)

New soils often get better with age, and gardeners will talk about soil mixes "mellowing" over the first six to eight months after they are put in place.

Get It Growing is a weekly feature on home lawn and garden topics prepared by experts in the LSU AgCenter. For more information on such topics, contact your parish LSU AgCenter Extension office or visit our Web site at www.lsuagcenter.com. A wide range of publications and a variety of other resources are available.